When an image forming apparatus which forms an image by an electrophotographic process adopts a direct transfer system of transferring an image by bringing a transfer member into contact with a photoconductor, the transfer member uses a conductive rubber roller having a conductive rotating shaft. The transfer member is driven to rotate to match the process speed of the photoconductor.
A voltage applied to the transfer member is a DC bias voltage. At this time, the polarity of the DC bias voltage is identical to that of a transfer voltage for general corona discharge. To achieve satisfactory transfer using the transfer roller, a voltage of generally 3 kV or more (required current is several μA) must be applied to the transfer roller. This high voltage necessary for the above image forming process is conventionally generated using a wire-wound electromagnetic transformer. The electromagnetic transformer is made up of a copper wire, bobbin, and core. When the electromagnetic transformer is used in application of a voltage of 3 kV or more, the leakage current must be minimized at each portion because the output current value is as small as several μA. For this purpose, the windings of the transformer must be molded with an insulator, and the transformer must be made large in comparison with supply power. This inhibits downsizing and weight reduction of a high-voltage power supply apparatus.
In order to compensate for these drawbacks, it is examined to generate a high voltage by using a flat, light-weight, high-output piezoelectric transformer. By using a piezoelectric transformer formed from ceramic, the piezoelectric transformer can generate a high voltage at higher efficiency than that of the electromagnetic transformer. Electrodes on the primary and secondary sides can be spaced apart from each other regardless of coupling between the primary and secondary sides. Thus, no special molding is necessary for insulation, and the piezoelectric transformer brings an advantage of making a high-voltage generation apparatus compact and lightweight.
A conventional high-voltage power supply circuit using a piezoelectric transformer will be explained with reference to FIG. 8. In FIG. 8, reference numeral 101 denotes a piezoelectric transformer (piezoelectric ceramic transformer) for a high-voltage power supply. An AC output from the piezoelectric transformer 101 is rectified and smoothed to a positive voltage by diodes 102 and 103 and a high-voltage capacitor 104, and supplied to a transfer roller (not shown) serving as a load. The output voltage is divided by resistors 105, 106, and 107, and input to the non-inverting input terminal (positive terminal) of an operational amplifier 109 via a protection resistor 108. The inverting input terminal (negative terminal) of the operational amplifier receives, via a resistor 114, a high-voltage power supply control signal (Vcont) which serves as an analog signal and is input to a connection terminal 121 from a DC controller 201.
The operational amplifier 109, the resistor 114, and a capacitor 113 are connected as shown in FIG. 8 to construct an integrating circuit. The control signal (Vcont) smoothed by an integral time constant determined by the component constants of the resistor 114 and capacitor 113 is input to the operational amplifier 109. The output terminal of the operational amplifier 109 is connected to a voltage-controlled oscillator (VCO) 110. A transistor 111 whose output terminal is connected to an inductor 112 is driven to apply a power supply voltage to the primary side of the piezoelectric transformer.
The high-voltage power supply unit of an electrophotographic image forming apparatus comprises a plurality of high-voltage power supply circuits using the piezoelectric transformer shown in FIG. 8. The high-voltage power supply circuits correspond to image forming units for, e.g., yellow (Y), magenta (M), cyan (C), and black (K), and output biases for charging, development, transfer, and the like.
An example of the above-described prior art is disclosed in, e.g., Japanese Patent Laid-Open No. 11-206113.
In the prior art, the output voltage is controlled by changing the driving frequency of the piezoelectric transformer. When the output voltage is controlled at a driving frequency higher than the resonance frequency f0, the piezoelectric transformer is driven at a frequency much higher than the resonance frequency, and a frequency serving as a voltage output stop point is controlled in a range where the frequency is much higher than the resonance frequency. Even when no voltage is output from the power supply apparatus, the piezoelectric transformer is driven at a driving frequency much higher than the resonance frequency in the internal circuit of the power supply apparatus. A low output voltage is kept output from the power supply circuit. The voltage is always applied to process members such as the photoconductor and transfer roller, shortening the service life of the process members. Further, power is wasted by driving the piezoelectric transformer.